1. Field of the Invention
The present invention generally relates to duplexers, and more particularly, to a duplexer that utilizes surface acoustic wave (SAW).
2. Description of the Related Art
So as to separate transmission and reception signals from other signals, antenna duplexers with surface acoustic wave (SAW) filters are mounted in most mobile-phone handsets today. Each of the SAW duplexers includes a transmission SAW filter, a reception SAW filter, and a matching circuit that is interposed between the reception filter and the connecting point of the transmission and reception filters. This matching circuit is provided for guiding each transmission signal from the transmission filter toward an antenna terminal and for preventing each transmission signal from flowing into the reception filter. Normally, the matching circuit is formed with a distributed-constant line such as a microstrip line or a strip line, as shown in FIG. 1. A microstrip line or a strip line may be incorporated into the package in which the SAW filters are to be mounted, or may be formed on the printed board on which the SAW filters are to be mounted. As the mobile-phone handsets are becoming more and more sophisticated, there is an increasing demand for devices that are small in size and height. However, a conventional duplexer with a distributed-constant line cannot be made smaller or shorter, because a very long line is required as the distributed-constant line.
So as to solve this problem, Japanese Patent No. 3487692 and Japanese Unexamined Patent Publication No. 2004-228911 disclose the techniques of replacing the distributed-constant lines as the matching circuits with lumped-constant circuits. As shown in FIG. 2, instead of a line, a π-type circuit that is formed with a parallel capacitor, a series inductor, and a parallel capacitor is interposed between the reception filter and the connecting point of the transmission and reception filters. With this structure, a long line becomes unnecessary, and the device can be made much smaller and shorter.
However, by the technique using conventional lumped-constant circuits as matching circuits, the reactance elements such as inductors and capacitors are formed with bulk components (chip inductors or chip capacitors) or are incorporated into the package that contains SAW filters. With such a structure, the device cannot be minimized in size and height. On the other hand, U.S. Pat. No. 5,175,518 discloses a lumped-constant inductor that has a dielectric film formed on an insulating substrate such as alumina, and has a pattern formed on the dielectric film. U.S. Pat. No. 5,175,518 also discloses a capacitor that has a dielectric film interposed between electrodes on an insulating substrate. However, with a dielectric film being formed on an insulating substrate, the capacity coupling between adjacent inductors on the dielectric film becomes large due to the large dielectric constant of the dielectric film. As a result, the loss increases because of the resonance caused between the adjacent inductors, and the isolation characteristics deteriorate due to the coupling between the adjacent inductors. For this reason, the lumped-constant inductor and the capacitor disclosed in U.S. Pat. No. 5,175,518 are not suitable for the phase-matching circuits of SAW filters that need to exhibit low insertion loss in the band and high isolation characteristics. This presents the problem in producing a duplexer with excellent filter characteristics.